The present invention relates to a cooking top plate.
As a domestic or commercial cooking device, not only a conventional gas cooking device employing a cooking stove but also an infrared heating cooking device employing a radiant heater or a halogen heater and an electromagnetic heating (IH) cooking device have been used recently.
A top plate used for an electromagnetic heating cooking device, for example, has been made of a low-expansion material such as a glass, ceramics, or a crystallized glass requiring a small electromagnetic induction heating amount in terms of thermal efficiency, safety, and thermal shock.
Further, a gas cooking device which had conventionally employed a top plate made of enamel or stainless steel has been employing a top plate made of a nonmetal material such as a glass, ceramics, or a crystallized glass each having low thermal expansion because of excellent appearance and cleaning properties of the material.
An intended purpose of a cooking top plate is to prevent scattering of water, a seasoning, food, or the like to a heating unit. However, a recently demanded purpose thereof is to improve appearance of the cooking device by screening an internal structure of the cooking device such as a heating unit or wiring.
When a nonmetal material is used for a top plate, a first method of screening an internal structure of a cooking device by the top plate is to use a low-expansion crystallized glass dark-colored with a transition metal element such as a crystallized glass GC-190 (available from Nippon Electric Glass Co., Ltd.) and Ceran (available from SCHOTT AG) each having a brown color.
A second method thereof is to form a light shielding film on a surface of a substrate made of a transparent low-expansion glass through printing. For example, JP 10-273342 A describes a top plate provided with a light shielding film composed of a glass and an inorganic pigment on a surface of a substrate made of a transparent low-expansion crystallized glass. The light shielding film has a larger thermal expansion coefficient than that of the substrate made of a low-expansion crystallized glass. Thus, in JP 10-273342 A, a porous light shielding film suppresses formation of cracks in the film. Further, JP 07-017409 B describes a top plate provided with a lustered film composed of a noble metal and a base metal on a surface of a substrate made of a transparent low-expansion crystallized glass.
However, the above first method requires preparation of a glass substrate for each color in production of top plates having different colors, which increases production cost. Further, a transition metal element suitable for a glass is limited, which limits production of multicolored top plates. Further, a top plate for an electromagnetic heating cooking device often has an indicator for displaying an applied electric energy or the like provided on a side of a non-working surface (surface facing inside of a cooking device) of the top plate. However, a red light of the indicator can be barely seen from a side of a working surface (surface facing outside of the cooking device). When the indicator emits a light of a color except red, the light of the color is not displayed on the side of the working surface because the top plate does not transmit a light of any color except red. Thus, when the indicator emits a light of a color except red, the top plate must be perforated to display a light of the color on a side of the working surface, resulting in a problem of easily causing chipping or cracks in the top plate.
Further, the above second method conventionally has a problem of difficulties in handling of the light shielding film because the light shielding film has low abrasion resistance and is easily peeled off. In particular, a porous light shielding film composed of a glass and an inorganic pigment has very low abrasion resistance and must be provided on a side of a non-working surface of the top plate. When a temperature sensor is bonded onto a light shielding film by a heat resistant resin, the heat resistant resin infiltrates into the light shielding film, causing a problem in that a portion having the heat resistant resin bonded appears different when seen from a side of a working surface of the top plate. Further, the inorganic pigment or the noble metal constituting the light shielding film has a problem of coloring a glass or causing glass defects such as stones in recycling of the top glass.